This program project serves as the nucleus for the study of the biological aspects of aging at Boston University. The primary objective is to learn why animals age. Three systems are studied, the auditory system, the brain, and connective tissue. The research on the auditory system involves measurements of the actual hearing of rats, examination of their cochlea, and their brains. The brain studies also include other cortical areas and the olfactory bulb. The connective tissue studies are primarily of elastin synthesis and maturation in both intact vessels and by the cells of blood vessel wall in tissue culture. Under the scanning electron microscope there is loss with age of both receptor cells and inner and outer hair cells of the organ of Corti. In the larger III and V pyramidal cells of the corti there is a diminution in the number of spines along the apical and basal dendrites and a tendency for both the cell bodies and the nucleus to be smaller in older animals. Studies in the olfactory bulb have produced the rather surprising finding that the number of granule cells increase with age even in adult animals. In the nucleus of brain cells two new types of molecular associations have been demonstrated. Sodium borohydride reduction links have been found to link histones H4 and in H2 nucleosomes, while the structural protein of the nucleus dissociates less readily and in older animals sediments in the DNA fraction. The mechanisms of these phenomenon are being studied.